The present invention relates to a drive belt. The invention further relates to a method for producing a continuous band of such drive belt and to a continuously variable transmission wherein such drive belt is utilised.
Drive belts of the present type are generally known through their application in continuously variable transmissions intended for the transmission of mechanical power at continuously variable speed and torque ratios between an engine and a load in particular for automotive purposes. Such drive belts are also known from the patent specification EP-0.279.645 B1. The known drive belt generally comprises one or two endless rings and an array of plate-like transverse elements oriented mutually parallel transverse to a longitudinal direction of the drive belt, whereby the endless ring Is provided in a slot of the elements such that the elements may freely slide along the ring in the longitudinal direction thereof. The endless ring typically is laminated comprising a number of concentrically stacked continuous bands. Through this measure, the ring may have a considerable tensile strength, whereas it is still relatively easily bendable in its longitudinal direction.
Because of the nature of use in continuously variable transmissions, where it rotationally connects two pulleys each having two pulley discs that define a V-groove of variable width, the known drive belt is subjected to tensioning, bending and stretching during operation, resulting in high internal stress levels that vary in dependence on the rotational speed of the pulleys and the torque applied to the transmission. The trajectory of the belt thereby includes two longitudinally straight parts where it crosses over from one pulley to the other and two longitudinally bent parts where it runs between the discs of a pulley at a respective radius of curvature for each of the two pulleys, which radii define the transmission ratio of the transmission. As a result of the tensioning, bending and stretching, a tensile stress in the continuous band near its radially inwardly oriented surface and a tensile stress near its radially outwardly oriented surface varies cyclically between a maximum stress level and a minimum stress level during operation of the drive belt in the transmission. Such cyclical variations render the drive belt prone to fatigue cracking, which may ultimately cause the drive belt to break apart and fail. To minimise the risk of belt failure due to fatigue cracking, or put alternatively to extend functional belt life as much as possible by improving its resistance against fatiguing, the known continuous bands are pre-bent, i.e. they are provided with an internal residual stress distribution during manufacturing. According to the known art the internal residual stress distribution is provided such that during operation of the drive belt the maximum tensile stress near the radially inwardly oriented surface and the maximum tensile stress near the radially outwardly oriented surface of the continuous bands are equal and, consequently, that the overall maximum tensile stress is at a minimum. The above-mentioned situation occurs when the internal residual stress distribution of a continuous band corresponds to a stress distribution under the influence of which the continuous band would be longitudinally bent at a radius of curvature that is twice a minimum radius of curvature at which it may be bent during operation. The radius of curvature at which a continuous band would be curved under the influence of the internal residual stress distribution, e.g. when cut, is hereby denoted as a pre-bending radius. This relation between pre-bending radius and minimum radius of curvature accurately holds, particularly when a thickness of the continuous band as seen in the radial direction of the curvature is relatively small compared to the minimum radius of curvature, which is normally the case for the drive belt. It is remarked that it is known from EP0.283.303 B1 to determine such internal residual stress distribution of a continuous band by transversely cutting the continuous band and by measuring the radius of the curvature in the longitudinal direction of the posture assumed by the cut continuous band.
Thus according to the known art the desired pre-bending radius is defined as twice the minimum radius of curvature at which the endless ring is bent in its longitudinal direction during normal operation of the transmission in which the drive belt is applied. It is noted that generally speaking and at least for drive belts to be applied in passenger car transmissions, such minimum radius of curvature occurring during operation corresponds fairly accurately to a minimum physical radius of curvature of the drive belt that is determined by the transverse elements having a taper defining a maximum amount of mutual rotation of adjacent and mutually contacting elements about an axial of the drive belt in combination with a dimension of the elements in the longitudinal direction of the drive belt, alternatively denoted element thickness. Of course, the latter minimum radius is somewhat, though usually only slightly, smaller than the minimum radius of curvature actually occurring during operation to allow the full range of transmission ratios of the transmission to be realised. In practice, the optimum pre-bending radius of the continuous bands may be accurately approximated by increasing the minimum physical radius of curvature of the drive belt by about 10%, at least for typical automotive application of the drive belt such as in passenger cars.
Although pre-bent at such pre-bending radius the continuous bands should provide the drive belt with a longest possible functional life, it surprisingly appeared in practice that the drive belt is still prone to early failure with respect to what was to be expected theoretically. Accordingly, currently applied drive belts are over dimensioned with respect to their nominal torque transmission capacity, which means that they are provided with an endless ring or rings that has or have a larger longitudinally facing cross sectional surface area than that what would theoretically required according to the known art. Such increased cross sectional surface area favourably decreases the maximum stress level in the continuous bands, which may for instance be realised by increasing the number of continuous bands applied in a ring or by increasing the transverse width thereof. These measures, however, adversely affect the drive belt cost price and size and, therefore, are principally undesirable.
It is an object of the invention to improve the functional life of the known drive belt without increasing its cost price, or, alternatively, lowering the cost price of the drive belt for a given nominal torque transmission capacity. According to the invention this objects is surprisingly realised with the drive belt according to the below.
Extensivexe2x80x94fatiguexe2x80x94testing, both with assembled drive belts and with separate continuous bands running around two cylindrical rollers, and analysis of the results thereof surprisingly appeared to reveal that, at least in those cases where drive belt failure could be indisputably attributed to fatiguing of a continuous band, the drive belt functional life could be improved by adopting a pre-bending radius that is considerably larger than twice the minimum radius of curvature that occurs during operation, which was previously considered the optimal value. According to the invention, this phenomenon may be accounted for by the observation that in the known drive belts fractures appear to initiate more often near the radially inwardly oriented surface of the radially innermost continuous band of the ring than near the radially outwardly oriented surface. From these observations, it is hypothesised that as a result of he interaction between the transverse elements in the curved trajectory part of the drive belt and the radially inwardly oriented surface of the radially innermost continuous band the stress level near the radially inwardly oriented surface are elevated, e.g. as a result of locally introduced contact stresses. It is further hypothesised that the discrepancy between the known theory and the tested practice may be the result of the transverse elements causing contact stresses in the continuous band, which result from the elements being pushed radially outwards by the pulley against the endless ring that is thereby tensioned and which accordingly are predominantly localised near the radially inwardly oriented surface of the radially innermost continuous band. These contact stresses are superimposed on the tensile stress in the continuous band during operation of the drive belt due to the tensioning, bending and stretching and disadvantageously appear to locally increase the tensile stress levels. Particular in case of continuous bands made from a material containing non-metallic inclusions, such as for instance Titanium-Nitride (TiN) or titanium-Carbide (TiC) inclusions, which are normally contained in the materials such as managing steels that are presently applied for the continuous bands of drive belts, the phenomenon appears to be quite pronounced and critical. With such materials a fatigue fracture of the band that leads to the failure of the belt appeared to initiate not at its surface, which would normally be expected, but inside the band material though relatively close to the surface at the location of a non-metallic inclusion. Apparently, such inclusions, at least when they are of significant size, e.g. having a dimension larger than approximately 5 microns, locally raise the stress level in the continuous band above the nominal levels of the above-mentioned contact and tensile stresses.
By pre-bending at least the radially innermost continuous band of the endless ring in accordance with the present invention, the maximum tensile stress due to the bending and stretching is reduced near its radially inwardly oriented surface at the expense of an increased tensile stress near its radially outwardly oriented surface, which would appear to be undesirable, since by this measure the overall maximum stress level in the continuous band is also increased and a decrease in fatigue strength would consequently be expected. However, according to the invention in this manner the fatigue strength is in fact surprisingly increased, because the disadvantageous effect of the contact stresses near the radially inwardly oriented surface of the continuous bands are at least partly compensated for by the reduced tensile stress due to bending. Thus given the nature of the application of the continuous band in a drive belt for application in a continuously variable transmission, the measure according to the invention appears to more or less levels out the maximum tensile stress levels during operation. With this measure, the resistance against fatiguing of the innermost continuous band may be increased surprisingly without reducing the maximum allowable load on the drive belt during operation, i.e. without reducing its nominal torque transmission capacity.
It is remarked that it is generally considered particularly cost effective if all continuous bands in the ring have the same pre-bending radius, i.e. may be manufactured in the same manufacturing process using similar process settings. According to the invention this may indeed be allowed, because even though the other continuous bands of such drive belt are subjected to an increased maximum stress level near their respective radially outwardly oriented surface, the radially innermost continuous band receives additional stresses during operation of the drive belt, such as the contact stresses, and accordingly will still be the most prone to fatigue cracking. Of course, when a given maximum pre-bending radius is exceeded, the increased maximum stress level near the radially outwardly oriented surface of the continuous band may cause local fatigue crack initiation. A suitable maximum workable ratio between the maximum stress levels near the radially outer and radially inner surface respectively was found to be about 3. Below such value, the crack initiation appears to be more or less randomly distributed between the radially inwardly and outwardly oriented surfaces. Using the known equations that relate the maximum stress levels to the pre-bending radius, which are e.g. known from EP-B1-0.279.645, it may be calculated that the maximum stress ratio conform""s to a pre-bending radius having a value of about 4.0. Thus according to the invention the pre-bending radius preferably has a value in the range between 2.5 and 4.0 times the minimum radius of curvature of the endless ring that occurs during operation, preferably having a value about halfway the range, i.e. approximately 3.3 times the minimum radius of a longitudinally curved part of the drive belt. Adopting a still larger pre-bending radius was found to undesirably result in fatigue crack initiation near the radially outwardly oriented surface of the continuous band.
In this respect, it is noted that in the known belt during operation interaction also may occur between the radially outermost continuous band of the endless ring and the transverse elements. Such feature appears to be caused by the phenomenon that due to elastic deformation of the pulley discs at the location where the endless ring of the drive belt exits the V-groove, the transverse elements thereof tend to continue to rotate with the pulley until they are pulled away from the pulley by the endless ring, through interaction with the radially outermost continuous band thereof. This interaction causes contact stress that locally effect a higher load on the radially outermost continuous band, in particular near its radially outwardly oriented surface. Accordingly, the invention also relates to a drive belt provided with an endless ring comprising at least two concentrically stacked continuous bands, the radially outermost band of the endless ring being provided with a pre-bending radius that is slightly less than 2 times, preferably about 1.9 times the minimum radius of a longitudinally curved part of the drive belt.
If it is considered to be more preferable to adopt continuous bands in the ring all provided with the same pre-bending radius, the additional load on the outermost continuous band may still be taken into account by adopting a ratio between the maximum stress levels that is somewhat lower than the preferred value of 3.3 mentioned earlier, thereby decreasing the maximum stress level near the radially outer surface of the continuous bands. According to the invention, a pre-bending radius of about 3 times the minimum radius of a longitudinally curved part of the drive belt was in this respect found suitable.
In an alternative embodiment of the invention, the pre-bending radius of the continuous band varies along its circumference between a minimum pre-bending radius and a maximum pre-bending radius, preferably varying elliptically. Such a continuous band has a significant advantage in a preferable manufacturing process, which includes at least the process steps of pre-bending the continuous bands evenly along its circumference, which results in a more or less evenly distributed internal residual stress distribution, and subsequently heat-treating the continuous band. In the latter process step the continuous band needs to be reliably suspended in a furnace with its surface exposed as much as possible, so as to realise a complete and evenly distributed heat treatment. According to the invention, in fulfilling these requirements together with easy product handling it is highly advantageous to force the band in a more or less elliptical shape, such that its tendency to assume the circular shape that was realised in the pre-bending process step may be used to reliably hold the band in a suspension device. Moreover, the elliptical shape is preferably for reasons of process capacity, in particular in case of a (semi-) continuous or production process. Relaxation of the internal stresses that occurs during the heat-treatment will, as a consequence of this suspension method, not be uniform along the circumference of the continuous band and the pre-bending radius will thus vary along between a minimum pre-bending radius and a maximum pre-bending radius.
Departing from the known art and given a desired ratio between the maximum and the minimum pre-bending radii, it would be considered preferable that the actual maximum and minimum pre-bending radii lie mirrored on either side of the optimum value of twice the minimum radius of longitudinal curvature such that the maximum tensile stress near the radially inwardly oriented surface and the maximum tensile stress near the radially outwardly oriented surface of the continuous bands are again equal. Based on the above-mentioned insights and experiences, however, superior fatiguing properties are in this respect surprisingly obtained with the drive belt according to the below.
A further improvement may be realised if the smallest value of the pre-bending radii, which appears to be the most critical based on the insight that fatigue crack initiation in practice occurs predominantly near the radially inwardly oriented surface of the continuous band, is at least equal to 2 times the minimum radius of a longitudinally curved part of the drive belt. Thus, to advantageously realise optimum process handling of the continuous band at least without deteriorating its fatigue strength, according to the invention the smallest pre-bending radius of the ratio the continuous band is equal to or larger than twice the minimum radius of longitudinal curvature. According to the invention the ratio between the maximum pre-bending radius and the maximum pre-bending radius thereby is between 1.5 to 2.5, or, more preferably, about 2, so as to realise a desirable process handling while keeping the difference in the maximum stress levels along the circumference of the continuous band within an acceptable limit.
In a further elaboration of the invention, both the minimum pre-bending radius and the maximum pre-bending radius have a value in the range between 2.5 and 4 that was defined earlier. In this manner an optimal drive belt fatigue resistance may be realised, however, since the ratio between the pre-bending radii is about 1.6 at most, the process handling will be slightly less than optimal.